1. Field of the Invention
This invention relates to a one-piece snap-on plastic hinged closure, having a lower part and a lid, which are connected with each other with an integral joint in the area where the jacket walls of the lower part and the lid are superimposed on each other, as well as with at least one intermediate element which creates the snap effect.
2. Description of Prior Art
Snap-on plastic hinged closures of the type mentioned above are known in large numbers and multitudinous designs. Conventional plastic closures mainly include two basic elements. For one, they have a main joint around which the pivot movement of the lid in relation to the lower part takes place, and they furthermore have one or several intermediate elements creating the snap effect. Such intermediate elements can be in the form of straps, triangles or angled flexible springs or even longitudinally deformable tension spring elements.
The essential part of this invention is focused upon the design of the joint connecting the two parts, the lower part and the lid. This joint is typically embodied as a film hinge in one-piece, snap-on plastic hinged closures. In the completely closed or the completely opened state of the closure this film hinge is not subjected to a force. In all intermediate positions, compression and displacement forces are exerted on this film hinge. The formation of microscopic cracks and scores can be noted upon a close inspection. Stretching of the film hinge, as well as greatly spreading white fracture places can be clearly seen in the area of the film hinge. These conventional uniaxial hinges are stressed most in the range of the unstable equilibrium during each opening or closing operation. The forces generated by the intermediate elements do pull the two closure parts toward each other, but since they are not located vertically above each other in all intermediate positions which differ from the completely closed position, a reaction force is created which must be absorbed by the film hinge. If the tension forces are reduced in general, the life of the film hinge is extended, however, the snap effect of the closure is simultaneously and to a large extent lost.
The second problem with the design of the joint between the lower part and the lid is that the joint always projects out in relation to the jacket wall. That fact that this joint projects out in relation to the jacket walls is on the one hand the result of the geometry of the snap-on hinged closures and, on the other hand, it is done for reasons of manufacturing technology. The more the joint projects out in relation to the jacket wall, the greater is the snap effect of the closure and conversely this snap effect is reduced the less the joint projects out in relation to the jacket walls. Since customarily the snap hinges are injection-molded in the completely open position, a vertical wall of material remains in the injection mold below the joint. If the joint is designed to sit as closely as possible to the jacket walls, this wall of material in the injection mold becomes so thin that the service life of the injection mold is reduced and it becomes very prone to defects.